The present invention relates to a method and apparatus for measuring the orientation of a body in space with respect to the three orthogonal axes. Existing two-dimensional apparatus are commonly called autocollimators.
The most common configuration of autocollimator employs an expanded collimated light beam which is reflected off a flat mirror attached to the object. The reflected beam is focused by a lens, and the lateral displacement of the focal point is proportional to the angular displacement of the mirror (FIG. 1). Defining the optical axis (the z-axis) perpendicular to the reference plane of the mirror, the focal point deviations in the x- and y-directions indicate tilts of the mirror around the y-axis and x-axis, respectively.
The reflected beam cannot be used to detect angular displacement in the third dimension, namely rotation around the z-axis. To detect such an angular displacement, a second autocollimator must be used at a right angle to the first. This possibility is impractical when measuring at large distances, or when the object can be accessed from one direction only.